In the field of high density integrated circuit (IC) construction, it becomes necessary to photolithographically define electrical conductors having a very small cross section and a high structural strength and integrity in order to maximize the packing density of interconnected devices which may be constructed in an integrated circuit chip. In the past, these processes have involved various ways to etch or cut through a metal layer such as tungsten or aluminum deposited on top of a thin film substrate so as to define individual conductive strips of metal used to interconnect devices on an integrated circuit chip.
One prior art process used for this purpose involves first developing a dielectric etch mask such as silicon nitride, Si.sub.3 N.sub.4, to expose predefined areas of an underlying metal substrate and then using a plasma etchant to etch through the exposed portions of the metal substrate to thereby define individual conductive strips therein. Plasma etchers and etching systems and related chemical reactions employed by these systems are generally well known in the semiconductor processing arts and are described, for example, in my copending application Ser. No. 07/536,732 entitled "Method and Apparatus Useful In The Plasma Etching of Semiconductor Materials", filed June 12, 1990, assigned to the present assignee and incorporated herein by reference.
A significant disadvantage of using the above Si.sub.3 N.sub.4 process relates to the large number of processing steps required in order to ultimately define the individual conductive strips of metal. That is, it is first required to deposit a layer of silicon nitride on the metal substrate and then develop a photoresist mask thereon. Then, it is required to etch through areas of the silicon nitride layer exposed by the photoresist mask in order to define the geometry of the dielectric silicon nitride etch mask. Then the photoresist mask remaining on the dielectric Si.sub.3 N.sub.4 layer has to be removed therefrom before etching the exposed areas of the underlying metal substrate to define the individual conductive strips therein. Obviously, the requirement for this large number of individual semiconductor processing steps reduces achievable process yields and makes the process relatively expensive and complicated.
In an attempt to provide a process alternative which would eliminate some of the above prior art processing steps, it has been proposed to deposit only a photoresist mask directly on the metal substrate and thereby eliminate altogether the above intermediate dielectric mask layer formation. However, unacceptable lateral etching will occur if a wet etching system is used in combination with a photoresist mask formed directly on a metal substrate. Therefore, dry etchers must be used for this purpose, and in the past plasma dry etching systems have been used in combination with a photoresist mask formed directly on a metal substrate. This alternative process can result in significant anisotropic etching of the metallic substrate exposed by the photoresist mask. In addition, this plasma etching process has been made practical by changing from an etch having little resist selectivity to an etch having a greater resist selectivity and therefore more suitable to preventing unacceptable attack of the photoresist mask and undercutting beneath the photoresist mask.
Whereas the use of the above plasma etching process in combination with a metal substrate masked directly with a polymer such as photoresist has significant advantages over the previously used silicon nitride type of hard masking for both wet and dry etching systems, there is nevertheless still an undesirable amount of attack of the photoresist mask by the dry etchant chemicals used as well as lateral etching and undercutting beneath the photoresist mask during this dry etching process. This lateral etching takes place during the plasma etching process despite the fundamental anisotropic etching nature of the plasma etching process. This anisotropic etching means, among other things, that the etch rate in the vertical or "Y" dimension into the etched material is significantly greater than the etch rate in the lateral or "X" dimension, thereby producing generally V-shaped grooves in the etched material.
The above characteristics of undesirable resist attack, lateral etching, and undercutting in these prior art plasma etching systems produces mesa shaped areas in the plasma etched metal underlayer having non-uniform sidewalls. These metal sidewalls are sometimes etched and tapered in an inwardly direction beneath the flat tops of metal mesas which are protected by the photoresist mask. This resist attack, lateral etching or undercutting during the plasma etching process reduces the structural integrity of the individual metal conductors thus formed. This in turn makes it difficult to provide even and uniform step coverage over these conductors when they are formed on the surface of an integrated circuit and are there protected by glass passivation or other surface protective layers. This metal sidewall attack also makes it difficult to construct conductors having good consistent cross-sectional areas required for good and uniform electrical conduction.
In an effort to reduce the above undesirable lateral etching and undercutting of the conductor sidewalls during plasma etching to form aluminum conductors, silicon tetrachloride, SiCl.sub.4, has been added to the plasma reactants to thereby produce and deposit a silicon containing dielectric material on the side walls of aluminum conductors being formed and during the anisotropic plasma etching process. This aluminum sidewall protective layer in turn produces a retardation of the above undesirable horizontal or lateral etching and thereby reduces undercutting of the aluminum islands or aluminum conductors thus formed. However, the use of this latter process employing silicon tetrachloride as the silicon-containing material to form a sidewall inorganic layer for aluminum has not proven entirely satisfactory inasmuch as the deposition rate of the dielectric material formed on the aluminum side walls is too slow. Furthermore, the thin inorganic film produced by this SiCl.sub.4 process has not been sufficient in thickness and density to in fact prevent all of the above undesirable horizontal or lateral etching of the aluminum sidewalls during the conductor forming process as previously described.
In the art of dry etching tungsten layers masked directly with a photoresist mask to form individual conductors of tungsten, W, prior art processes have used either SF.sub.6, NF.sub.3, or Cl.sub.2 --O.sub.2 as the active etchant gas. This gas is introduced into the plasma reaction chamber in order to actively etch the tungsten, and as a byproduct it forms films of tungsten oxides and chlorides on the sidewalls of thus formed tungsten conductors. However, this prior art process often does not yield good high quality protective films of tungsten chloride with acceptable profiles. In addition, the chlorine gas, Cl.sub.2, has poor resist selectivity characteristics and undesirably attacks the photoresist etch mask on the surface of the tungsten. In fact, when chlorine gas, Cl.sub.2, is used as the active reactant and etchant gas in the dry etching process, it is normally reactived with oxygen in the reaction chamber. This produces an oxygen attack on a resist mask and thus makes it impossible to use a resist mask in this process. Instead, the use of a hard mask such as silicon nitride is required.
There is yet another prior art process which has been used to form sidewall passivation films during the plasma etching of metal layers to form conductive strips therefrom. This prior art process uses fluorocarbon polymers or chlorocarbon polymers in the reaction chamber to in turn to produce organic films rather than inorganic films as the sidewall passivation layer on the thus formed conductive layers. Suitable polymers for this purpose are tetrafluoromethane, chloroform, and methylchloride. However, these polymers have been found to be carcinogenic and, in addition, produce a great deal of undesirable and unacceptable deposition of organic materials in the gas reaction chamber. These materials have to be manually cleaned with much difficulty after each wafer processing and etching operation of the type described herein.